The present invention pertains to the field of distillation of multicomponent fluid mixtures, and in particular to distillation processes for separating a ternary mixture having components, A, B and C (mixture ABC) into three product streams each enriched in one of the components, where A is the most volatile component and C is the least volatile component.
To separate a ternary mixture ABC into almost pure components a distillation process must use at least two distillation columns. Five such distillation processes are well known in the prior art: direct sequence, indirect sequence, side rectifier, side stripper and fully thermally coupled columns. (For example, see page 711 of the book entitled "Separation Processes" by C. J. King, McGraw-Hill, 1981). Of all the prior art ternary distillation processes, the fully thermally coupled column system requires the least amount of heat duty ("Minimum Energy Requirements of Thermally Coupled Distillation Systems", Z. Fidkowski and L. Krolikowski, AICHE Journal, pages 643-653, volume 33, 1987). In spite of this attractive performance, the fully thermally coupled column system has not been used widely in commercial applications, in part because of operating problems.
FIG. 1 shows a fully thermally coupled column (FC) process. A feed mixture containing components A, B and C (stream 100) is fed to a first distillation column 110, where the feed stream is separated into two streams (122, 132) that are primarily binary mixtures. The liquid (stream 122) from the bottom of this distillation column is primarily a binary mixture composed of components B and C. Similarly, the vapor (stream 132) from the top of this distillation column is primarily a binary mixture composed of components A and B. Both of these primarily binary mixture streams 122 and 132 are fed to different locations of a second distillation column 120. A portion of the liquid from the bottom of a second distillation column 120 is recovered as C-enriched product stream 190, and another portion of this liquid is boiled in reboiler 182 and returned as vapor stream 192 to provide boilup for the second distillation column 120. The vapor from the top of second distillation column 120 is condensed in condenser 112, and a portion is recovered as A-enriched product stream 170 while the other portion (stream 172) is returned to provide the needed liquid reflux for this distillation column. The B-enriched product stream 180 is produced from an intermediate location of second distillation column 120. (A location of a distillation column is an "intermediate location" when there is at least one separation stage above and one separation stage below that location. A "separation stage" is a mass transfer contact device between liquid and vapor phases, such as a suitable mass transfer tray or a packed height of a suitable packing.) This withdrawal location (intermediate location) is in between the feed locations of the two primarily binary feed streams 122 and 132.
The first distillation column 110 does not use a reboiler or a condenser. The boilup at the bottom of this column is provided by feeding a vapor stream 127 from the second distillation column 120. It is important to note that the withdrawal location of vapor stream 127 is from the same location of the second distillation column 120 as the feed location of the primarily binary liquid stream 122. This leads to a two-way communication between the two distillation columns. (In a two-way communication mode, when a vapor stream is sent from one column to another column, then a return liquid stream is implemented between the same locations of the two columns.) Similarly, the liquid reflux stream 137 to the top of the first distillation column 110 forms another two-way communication between the two distillation columns since stream 137 is withdrawn from the second distillation column 120 at the same location as the feed location of vapor stream 132. It is taught in the prior art that two two-way communications are needed to achieve the lowest heat demand for ternary distillation.
While the heat demand for the process in FIG. 1 with two two-way communications is lowest, it rarely has been used. The lack of use has often been attributed to fear of control problems ("Thermal Coupling for Energy Efficiency", H. Rudd, Supplement to the Chemical Engineer, pages S14-S15, Aug. 27, 1992; "The Design and Optimization of Fully Thermally Coupled Distillation Columns", C. Triantafyllou and R. Smith, Trans. IChemE, pages 118-132, Volume 70(A), 1992). One of the often cited concerns is the flexibility to control the flows over a wide range both at the top and bottom ends of the first distillation column 110. For the vapor AB in stream 132 to flow from first distillation column 110 to the second distillation column 120, it is required that the pressure at the top of the first distillation column 110 be greater than the pressure at the feed point of stream 132 in the second distillation column 120. At the same time, for the vapor BC in stream 127 to flow from the second distillation column 120 to the first distillation column 110 it is necessary that the pressure at the bottom of first distillation column 110 be lower than the pressure at the originating point of stream 127 in the second distillation column. This leads to an unique restriction that the pressure at the bottom of the first distillation column 110 be lower than the pressure at a point in the bottom section of the second distillation column 120, and at the same time, the pressure at the top of the first distillation column must be higher than the pressure at a point in the top section of the second distillation column. This requires careful adjustment of pressures in both of the columns and presents operating concerns for plants requiring wide ranges of variation in flow rates and other operating parameters. C1 early, there is a need for alternative column arrangements with higher operating flexibility while maintaining lower heat demand for distillation.
It is worth noting that both of the liquid transfer streams 137 and 122 at the top and bottom of the first distillation column 110 flow in a direction opposite of the flow of the vapor streams (122, 132) at each end. This requires that either a pump be used on each of the liquid streams or that the relative heights of the two columns be adjusted to allow each of the liquid streams to be transferred through gravity.
Recently, Agrawal and Fidkowski introduced the process shown in FIG. 2 (U.S. Pat. No. 5,970,742) In this figure, the bottom end of first distillation column 110 has a two-way communication with the bottom section of second distillation column 120 and the top end of the first distillation column 110 has only one-way communication with the top section of the second distillation column 120. Thus, liquid stream 122 from the bottom end of the first distillation column 110 is sent to the bottom section of second distillation column 120. A vapor stream 127 is withdrawn from the second distillation column 120 and sent to the bottom of the first distillation column 110. A portion of the vapor stream exiting from the top end of the first distillation column 110 is sent as stream 132 to the second distillation column 120. Unlike FIG. 1, there is no liquid return stream to the top of the first distillation column from the second distillation column. Instead, a portion of the vapor stream (stream 234) from the top of the first distillation column 110 is condensed in condenser 215 and returned as liquid reflux in line 236. Once again, a vapor stream (stream 132) is transferred from the first distillation column to the second distillation column and a second vapor stream (stream 127) is transferred in the reverse direction. This leads to some of the same operating challenges as described for the process in FIG. 1.
Recently, Agrawal and Fidkowski (U.S. Patent Application Serial No. 6,106,674) suggested the use of an alternative equivalent configuration of FIG. 3 to solve the problem of careful control of pressure profiles in the distillation columns of the configuration in FIG. 1. In this solution, the distillation section 6 and the associated reboiler 182 from the second distillation column 120 (of FIG. 2) is moved to the first distillation column 110 below the distillation section 2 in FIG. 3. This leads to a two-way connection in which vapor stream 350 enriched in components B and C is transferred from the first distillation column 110 to the bottom of the second distillation column 120, and a liquid stream 352 is returned from the bottom of the second distillation column 120 to the first distillation column 110. Note that now both the vapor streams 132 and 350 are transferred from the first distillation column to the second distillation column. Therefore, the first distillation column 110 can be operated at a pressure higher than that of the second distillation column 120, and the flow of vapor streams 132 and 350 can be controlled by using control valves in either one or both of the lines transporting those streams.
While the configuration in FIG. 3 is more operable than the one in FIG. 1, the presence of vapor flow between the columns is less desirable. Generally, this vapor flow is very sensitive to the pressure drop in the lines. Therefore, the pressure drop across the control valves or the pressure difference between the columns have to be carefully monitored and controlled.
There are other ternary distillation processes with only one two-way communication between the two distillation columns. The two well-known processes are side stripper and side rectifier. The side stripper configuration is shown in FIG. 4. This prior art process can be easily derived from the configuration shown in FIG. 3. Now distillation section 3 and the associated condenser 112 is located on top of distillation section 1 of the first distillation column 110. However, the second distillation column 420 has only section 4. Distillation section 5 is eliminated and in its place, reboiler 440 is used to provide the boilup to the second distillation column 420 (also known as a side stripper column). Component B is recovered from the bottom of the second distillation column 420. Vapor and liquid streams 433 and 438 establish the two-way communication between the distillation columns.
Another ternary process with one two-way and one one-way communication between the two distillation columns suggested by Agrawal and Fidkowski (U.S. Pat. No. 5,970,742) is shown in FIG. 5. The difference between this figure and the prior art process of FIG. 1 is that no vapor stream 127 is transferred from the second distillation column 120 to the first distillation column 110. Instead, a portion of the liquid stream from the bottom of the first distillation column 110 is boiled in the reboiler 528 and fed to the bottom of the first distillation column as stream 526. Note that there still is a vapor transfer stream 132 between the two distillation columns.
The same challenge exists when mixtures containing more than three components are distilled to produce product streams each enriched in one of the components. The reason being that the distillation processes with low heat demand used to distill mixtures with more than three components are made up of multiple two-way and one-way communications between the distillation columns. Therefore, deficiencies of the ternary subprocesses also are carried to the distillation of mixtures containing a greater number of components. Some known examples of four and five component distillation processes are described in a paper by Agrawal ("Synthesis of Distillation Column Configurations for a Multicomponent Separation", Ind. Eng. Chem. Res., volume 35, pages 1059-1071, 1996) and a paper by Sargent ("A Functional Approach to Process Synthesis and its Application to Distillation Systems", Computers Chem. Eng., volume 22, pages 31-45, 1998).
A sequential four-component separation process with at least four two-way communications is shown in FIG. 6. The feed mixture ABCD is distilled into four product streams (170,180, 660, 690). In this mixture, the relative volatility follows the alphabetical order, i.e., A is the most volatile, D is the least volatile and B is more volatile than C. The first distillation column 610 has two two-way communications with the second distillation column 620 which in turn has at least two (generally three) more two-way communications with the third distillation column 630. C1 early, the challenges associated with the vapor transfers between the distillation columns now are much greater as the pressure profiles in all of the three distillation columns must be carefully controlled.
A four-component separation process with a dual satellite column arrangement is shown in FIG. 7. The feed mixture ABCD is fed to the main column (first distillation column) 710. There are two satellite columns (720, 730) each with two two-way communications with the main column. There is shown a possibility of having a liquid and vapor flow between the two satellite columns. The product stream 170 enriched in the most volatile component A is produced from the top of the main column 710 and the product stream 790 enriched in the least volatile component D is produced from the bottom of this column. Product streams (760, 780) enriched in components of intermediate volatility (C, B) are produced from each of the satellite columns (720, 730). For each satellite column, in order to transfer vapor streams between columns, the pressure at the bottom of the satellite column must be lower than the pressure at a point in the bottom section of the main column and simultaneously, the pressures at the top of the satellite columns must be greater than the pressure in the top corresponding section of the main column. Furthermore, pressures of each of the satellite columns have to be adjusted to allow the flow of vapor and liquid streams enriched in component B and C in proper direction between the two satellite columns. All this presents a great deal of difficulty in the operation of such integrated processes with multiple two-way communications between the distillation columns.
In a typical cryogenic air distillation system recovering oxygen and argon products, air at a high pressure is fed to a high pressure column. In the high pressure column the air is separated into liquid nitrogen at the top and an oxygen-enriched bottoms liquid known as crude liquid oxygen. This crude liquid oxygen, which is primarily a ternary mixture of nitrogen, argon and oxygen, is fed to a low pressure column. In the low pressure column, crude liquid oxygen is separated into nitrogen at the top and oxygen at the bottom. An argon enriched vapor feed is withdrawn from an intermediate location of the low pressure column below the crude liquid oxygen feed and fed to a side rectifier. An argon product stream is produced from the top of the side rectifier and liquid from the bottom is returned to the low pressure column. For this process, with the objective to reduce energy consumption, Erickson in U.S. Pat. Nos. 4,817,394 and 4,854,954 suggested a number of modifications.
In FIG. 4 of U.S. Patent No. 4,817,394, Erikson provided the condensing duty to the argon producing column at the top and at an intermediate height by vaporizing crude liquid oxygen in two stages. This vaporization of crude liquid oxygen in two stages produces two vapor streams of different oxygen concentrations that are fed to two different locations of the low pressure column. Furthermore, the argon producing column is extended by adding an additional section at the bottom of this column and a liquid stream is withdrawn from the low pressure column and fed to this column. The argon column is operated at a pressure significantly lower than the low pressure column and boilup at its bottom is provided by condensing nitrogen from the top of the high pressure column. Unlike the conventional oxygen and argon producing distillation columns, the bottom of the low pressure column is now boiled by either a partial or a total condensation of a portion of the feed air stream. U.S. Pat. No. 4,817,394 states that the key to the feasibility of these triple pressure configurations is that two vapor streams are fed to the low pressure column. In the configuration of FIG. 5 of U.S. Pat. No. 4,817,394, where one vapor stream is fed to the low pressure column, the second vapor stream is produced in the low pressure column by the condensation of a vapor stream from an intermediate height of the argon producing column. Both U.S. Pat. Nos. 4,817,394 and 4,854,954 insist that the argon producing column be operated about 1/3 to 1/2 bar lower in pressure than the low pressure column. This decreases the pressure of the feed supply air and leads to savings in energy consumption. However, condensation of a portion of the feed air at the bottom of the low pressure column deprives the distillation column of nitrogen reflux and leads to lower argon recoveries.
Subsequently, in U.S. Pat. No. 5,577,394, Rathbone described a flowsheet that essentially is similar to the one described in U.S. Pat. Nos. 4,817,394 and 4,854,954. The only significant difference is that Rathbone provides the second vapor to the low pressure column by condensing a portion of the nitrogen vapor from the top of the high pressure column at an intermediate height of the low pressure column. Once again, the objective of this patent is the same as the other two cited patents, i.e., to save energy.
None of the cited patents have tried to solve the general problem of either reducing or eliminating the vapor transfer between the columns in the thermally coupled columns of the prior art processes to improve their operability. In the thermally coupled columns such as the ones shown in FIGS. 1 through 7, the vapor flows are sensitive to pressure differences between the columns. This problem is particularly more challenging for the thermally coupled configurations with two or more connections between any two distillation columns wherein at least one of the connections is a two-way communication.
It is desired to have more operable multicomponent distillation processes having efficiencies that are similar to or better than the efficiencies of the prior art thermally coupled processes.